1. Field of the Invention
This invention relates to the field of hydrometallury. More specifically, the invention is directed to a lixiviation apparatus and method for applying a leaching solution to crushed ore.
2. State of the Art
An approach of common usage in extracting metals from ore is the technique known as percolation. In this technique a leaching solution is applied to a stationary mass of crushed ore. The solution interacts with specific metals contained in the ore to reduce a metal ladened solution, the metals being contained in the solution in the form of a salt. The solution is then removed from the ore mass. The solution is subsequently processed to precipitate the metal.
Various factors influence the rate at which the metal is removed from the ore mass subsequent to the introduction of the leaching solution to the ore. Notably, the quantity of leaching solution being distributed over the ore, the strength of that solution, the depth of the pad on which the ore is positioned and the particle size of the ore will all effect the rate of removal of the metal from the ore being treated.
In the conventional approach, a mass of ore is spread over an impervious surface known as a "pad." A pad may be formed by sheets of an impervious material, such as plastic. Alternative pad constructions have employed asphalt and compacted clay. It is also common that the pad be constructed on a slope. Adoption of a sloped configuration assists the processor in recovering the metal ladened solution subsequent to its passage through the ore mass.
Recently, efforts have been directed to methods of applying the leaching solution to the ore mass. It has been recognized that prior methods have tended to result in inexact applications of the solution to the ore mass. Of further concern has been the evaporation and degradation of the leaching solution which results from large applications of the solution to an ore mass in relative short periods of time.
Within the last several years that has been a concerted effort, primarily in South America, to leach ore on On-Off pads. This entails moving the ore to be processed on a pad, leaching the ore for 45 to 90 days and then subsequently removing the ore and discarding it in a dump site. In this technique, any metal remaining in the volume of unleached ore remaining on the pad prior to the removal of the ore is discharged with the treated ore resulting in a loss of potentially recoverable metal. In conventional practice the ore mass on the pad may reach depths as shallow as 2.4 meters to as deep as 7.6 meters. The use of On-Off pads has caused considerable concern in the industry in view of the potential for the considerable loss of metal in unreacted ore. It has been determined that the shallower the ore placement on the pad the higher the percentage of metal loss per leach cycle.
One recent effort of note in this area is disclosed in U.S. Pat. No. 5,005,806 (Krauth). In the Krauth disclosure, a arrangement of piping is positioned over and/or through the ore mass. The piping includes a number of laterally-spaced pipes which are fitted at spaced intervals along their respective lengths with emitters. These emitters are of the type commonly associated with drip irrigation. This arrangement produces a grid-like array of piping with each emitter being positioned at a uniform spaced distance from adjacent emitters. The Krauth system addresses many of the problems conventionally associated with percolation treatment of ores. Notably, the leaching solution is applied is relatively small quantities over time which tends to alleviate the concerns about premature evaporation of the solution and its degradation by ultraviolet rays.
While the Krauth system addresses many of the concerns which have been raised by those skilled in the art, it has been noted that the use of the Krauth system results in many areas of the ore mass remaining untreated by the leaching solution. Specifically, as shown in FIGS. 1 and 2 of the drawings, the natural flow patterns of the leaching solution from the emitters produces a generally upright cone shaped distribution pattern. Little, if any of the leaching solution, is directed laterally from the emitter to reach those areas of the ore mass which lie between adjacent emitters and near the surface of the ore mass. It follows that the flow patterns of the leaching solution in conjunction with the positioning of the emitters produces a number of "dry cone" shaped areas which remain untouched by the leaching solution. It should be recognized that being untouched by the leaching solution, these dry cones are not processed for the purpose of removing concentrations of metals which may be present in these areas.
It has also been discovered that the use of emitters for the purpose of applying leaching solution also presents problems for maintaining the proper operation of the emitters. Due to the relatively slow flow velocities encountered in the supply piping to the emitters, the small size of the nozzles found in conventional emitter structures, the small diameters of the tubing conventionally utilized with such emitters and the nature of the leaching solution the emitters tend to become clogged within relatively short periods of time resulting in very poor uniformity in the application of the leaching solution to the ore mass. It therefore becomes necessary for steps to be taken to unclog the emitters and thereby continue operation of the percolation system. Understandably, these cleaning operations disrupt the processing time of the ore mass.
The instant invention is directed to an apparatus and method for use in addressing the above identified concerns.